This invention relates generally to oven control. In particular, the invention relates to control of radiant heating elements in the heating section of a blow molding machine where precise temperature control of preforms to be blow molded is essential to producing high quality finished products.
It is known to use radiant heat sources in the temperature conditioning section of blow molding machines where hollow thermoplastic preforms are heated to raise the temperature thereof to a level at which the preform material exhibits properties suitable for blow molding. An inherent advantage of radiant heating is that energy from the source is absorbed and transmitted by the material to be heated resulting in a more efficient conversion of energy to heat within the material as compared to convection or forced air heating. To achieve these benefits of radiant heating, the radiant heat sources must be operated at temperatures at which substantial quantities of energy are emitted at wave lengths within the absorption and transmission bands of the material to be heated. Nevertheless, conversion of this energy to heat at the surface of the material causes surface heating to proceed more rapidly than heating within the interior of the preform wall section. If this surface heating is permitted to proceed without intervention, material degradation will occur before the interior material has been raised to the desired temperature. Therefore, it is necessary to provide a means of controlling the overall heating operation to maximize the benefits of radiant heating while at the same time preventing surface overheating.
The prior art teaches the application of a heat exchanging gas to the surface of the preform while heating continues. This method has the disadvantage of removing heat from the preforms and adding complexity to the machine heating section. A preferable approach is to operate the radiant heat sources with alternating predetermined periods of high and low level energization to permit heat produced at the article surface during high level periods to be conducted inwardly thereof during low level periods. This control scheme has the further advantage that it provides a relatively simple means for accomodating variations of wall section thickness over the heated portion of the article. Specifically, by arranging the radiant heat sources so that a particular source is associated with a particular segment of the article surface, the independent adjustment of periods for that source provides control of the heat produced within the associated preform wall section segment. Selectable or programmable heat time control has the drawback that the total energy produced during the heating cycle is susceptible to variations of the incoming voltage supply. Thus, where the supply voltage is high or low compared to the nominal value total energy delivered to the radiant heat sources, and consequently, the total energy radiated to the articles, varies. These variations may thus randomly produce preforms which are inadequately heated or overheated and which result in unacceptable final articles. In previously known controls, regulation of power in the heating elements against supply variations required either temperature sensing and feedback or some type of analogue regulating circuitry such as conduction phase control. Both of these solutions add significant cost to the total heating control. Further, where conduction angle or phase control is used, one result of chopping cycles of the supply voltage is the production of high frequency noise at substantial power levels reducing the overall system efficiency.
It is, therefore, an object of the present invention to provide an apparatus for controlling the total electrical energy delivered to a load during a predetermined period of time from an alternating current source producing a voltage subject to variations from a nominal value.
It is a further object of the present invention to provide a programmable period cycle control for radiant heating ovens having means for monitoring the supply voltage and efficient means for regulating total power delivered to the heat sources.
It is a further object of the present invention to provide a programmable period cycle control for radiant heating ovens having means for monitoring an alternating current supply voltage and means for selectively applying cycles thereof to radiant heat sources to regulate the total power delivered to the radiant heat sources during the heating cycle.
It is a still further object of the present invention to provide a programmable period cycle control for radiant heat ovens having independently programmable high and low power periods for each of a plurality of radiant heat sources and means for monitoring an alternating current supply voltage and means for selectively applying cycles thereof to the radiant heat sources according to a duty cycle ratio and prediction algorithm to produce a predetermined total energy emission from each heating element during a heating cycle in accordance with a difference between the actual measured voltage and a predetermined minimum voltage.
Further objects and advantages of the present invention shall become apparent from the accompanying drawings and the following description thereof.